Getchar and ungetc equivalents in zig?

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1

Does zig have functions that are equivalent to C library’s getchar() and ungetc(ch1, stdin)?

2

readByte is the getchar() equivalent. You can get a std.io.Reader using the reader() method of the file or network stream.

There is no ungetc in the standard library. You can use a char: ?u8 (optional byte) to implement ungetc like:

var char: ?u8 = null;

fn ungetc(ch: u8) void {
    char = ch;
}

fn getchar(reader: Reader) !u8 {
    if (char) |ch| {
        char = null; 
        return ch;
    }
    return reader.readByte();
}

Welcome to ziggit :slight_smile:

1 Like
3

char = ch?..

4

Yes, thank you.

5

Hey, that’s actually a pretty cool solution. The global variable makes me slightly worried, but I’ve got some ideas how to work around it.

The only really limitation is that it would only allow a single character ungetc (but that would not be something that I would need right now at least).

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6

This “buffer” can be placed into some struct with getc/ungetc methods or so.

BTW, here is a snippet from man ungetc:

ungetc() pushes c back to stream, cast to unsigned char, where it is
available for subsequent read operations. Pushed-back characters will
be returned in reverse order; only one pushback is guaranteed.

So “one-byte push-back buffer” is quite a “legal” solution.

7

That’s interesting. I never tried to ungetc more than one character, but I somehow assumed that it would work just fine if I did push back more. It’s always a good idea to read the spec/man, etc.

8 
const std = @import("std");

const CharReader = struct {
    char: ?u8,
    reader: std.io.Reader,

    fn init(reader: std.io.Reader) CharReader {
        return .{
            .char = null,
            .reader = reader,
        };
    }

    fn ungetc(self: CharReader, ch: u8) void {
        self.char = ch;
    }

    fn getchar(self: CharReader) !u8 {
        if (self.char) |ch| {
            self.char = null; 
            return ch;
        }
        return self.reader.readByte();
    }
};
1 Like
9

This is exactly what I was looking for! Thanks!

(I’m very new to zig and there are many “d’oh” moments when I crack my brain to figure out the correct way… and then I see the solution and it elegant and simple)

1 Like
10

It won’t, but you can use a stack (can be implemented using generic ArrayList from standard library, for ex.). If pop operation returns null, do actual reading.

Off-topic. I personally never used ungetc(). How can it be useful at all? Multiple input handlers?.. I searched a bit and found this.

I can’t find any useful examples of how this function could be necessary

Just out of curiosity, what are you using ungetc() for?

11

Maybe it is not the most elegant solution, but I sometimes use it for key detection.

  • ESC = 27
  • UP_ARROW = 27[A
  • A = A

So, if I want to detect both ESC, UP_ARROW and A, etc, I must “ignore” the first ESC and read the next character. If the next character is not [, then I can ungetc and continue the loop to capture it as another key that was pressed after normal ESC.